Method for calibrating a radiation detection system

ABSTRACT

An automatic and intuitive calibration method is provided for calibrating a radiation detection system for processing counts from a known radiation source. A probe is positioned near the radiation source, which may be the surgical injection site of a radionucleide. A control unit generates audible and/or visual feedback signals to cue the operator as to where to position the probe relative to the radiation source, in order to obtain a pulse frequency (count rate) in an optimal range for processing. The control unit then automatically identifies a peak energy level and sets an energy acceptance window having a mathematical relationship to the peak energy level, whereby the radiation detection system thereinafter processes only counts corresponding to energy levels falling within the energy acceptance window.

[0001] The present application is related to U.S. patent applicationSer. No. 09/266,961 filed on Mar. 12, 1999, which is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates, in general, to methods forprocessing signals from nuclear uptake probes for radiation detectionand, more particularly, to methods of adjusting a control unit tocalibrate a radiation detection system each time a new probe is used.

BACKGROUND OF THE INVENTION

[0003] Radioactive pharmaceuticals used in combination with radiationdetection systems have been proven to be effective in locating radiolabeled tissue within patients. These pharmaceuticals are also known asradionucleides and include solutions of Iodine 125, Iodine 131, andPhosphorous 32. Other radionucleides include monoclonal antibodies,peptides, and certain colloids labeled with radioactive isotopes such asTechnetium-99. Once a radionucleide is introduced into a patient's body,it will tend to collect at targeted tissue sites, such as, for example,lymph node sites and such sites may be located by looking forconcentrations of the radionucleide.

[0004] The mammalian lymphatic system has various interrelatedfunctions, including circulating and modifying tissue fluid formed incapillary beds and removing cell debris and foreign matter. For certaincancers, neoplastic cells migrate and collect at regional nodes withinan associated lymph drainage basin. Some cancers, such as thoseencountered in the breast, will evidence somewhat predictable nodalinvolvement. The axillary lymph node region is the principal site ofregional metastasis from carcinoma of the breast, and approximately 40%of patients have evidence of spread to the axillary nodes. In someapproaches to the disease, these axillary nodes are removed as a form oftherapy.

[0005] Sentinel node biopsy is a less invasive alternative to lymph nodedissection in diagnosing metastasis of breast cancer tumors. Theprinciple of sentinel node biopsy is that neoplastic cells detachingfrom the primary tumor are most likely to be held by the sentinel node,which is the first lymph node to receive lymph from the involved areaand the most likely site of early metastasis. If the sentinel node isfree of cancer, it is highly probable that all of the other nodes arefree of cancer cells. This knowledge helps the physician in staging thedisease.

[0006] Thus, it is important to identify the sentinel node when tryingto determine whether cancer has metastasized. Detection of a sentinelnode may be achieved by using a gamma ray detection probeintra-operatively to assist surgeons in locating tissue tagged with aradionucleide. U.S. Pat. No. 5,732,704 to Thurston et al discloses aradiation based method for locating and differentiating sentinel nodes.The method described is used to identify a sentinel lymph node locatedwithin a grouping of regional nodes at a lymph drainage basin connectedto neoplastic tissue. A radionucleide is injected near the neoplastictissue and migrates along a lymph duct toward the drainage basincontaining the sentinel node. A hand-held, radiation detection probe ismoved along the lymph duct while the operator observes a graphicalreadout of count rate amplitudes to determine when the probe is alignedwith the duct. The region containing the sentinel node is identifiedwhen the count rate at the probe substantially increases. Followingincision, the probe is maneuvered using a sound output to establishincreasing count rate thresholds. The probe is then moved incrementallyuntil the probe is adjacent to the sentinel node, which then may besurgically removed. The visual and audio signals used by the surgeon aregenerated by the signal processing portion of the radiation detectionsystem, which may be referred to as the control unit control unit. Thecontrol unit is connected to the handheld probe to form the radiationdetector system.

[0007] The success of using a method such as disclosed in U.S. Pat. No.5,732,704 depends upon the reliability of the hand-held radiationdetection probe and the calibration between the probe and the controlunit. The probe generally operates at room temperature and is designedto detect very low levels of gamma radiation. The gamma radiationemitted from the sentinel node may be masked by background noise such ascosmic radiation, thermal noise, and capacitively or piezoelectricallyinduced noise resulting from manipulation of the probe itself. Onefunction of the control unit is to filter gamma radiation emitted by theradio tagged tissue from background noise and other sources of gammaradiation, including Compton scatter.

[0008] Gamma ray detection probes may include a high-Z semiconductor(such as CdZnTe or CdTe) or a scintillation crystal such assodium-iodide (NaI) which is coupled with a small photo multiplier tube.U.S. patent application Ser. No. 09/066,545, filed on Apr. 24, 1998,describes a relatively low-cost radiation detection probe. The probeintegrates a silicon photodiode detector (with or without ascintillation assembly) with amplifiers, interface electronics, andradiation shielding, into one compact radiation probe assembly. Theprobe assembly uses relatively low voltages, has relatively fewelectrical connections, is relatively easy to manufacture, and islow-cost. The disclosed radiation detection probe is particularly usefulfor detecting radionucleides during lymphatic mapping and localizationof a sentinel node.

[0009] Despite recent advances to lower the cost of manufacture and useof radiation detection probes, it is still necessary to insure that theelectronic signal generated by the probes are correctly interpreted bythe control unit. Careful attention to manufacturing tolerances and theuse of specially selected electronic components may ensure adequatecalibration between probes and adequate stability after the probes leavethe factory, thus ensuring that the output for a given input isrelatively constant across a selection of probes and relatively stableover time. Of course, such manufacturing tolerances and specialelectronics add significant cost. Lower cost probes, on the other hand,may be manufactured to wider tolerances and utilize less expensiveelectronics, making them less consistent probe to probe and more likelyto lose calibration after leaving the factory. One particularlyimportant characteristic of radiation detection probes is the signaloutput level generated by a predetermined signal input level. Forexample, one probe may generate an electronic pulse output of 5.1 voltswhen a gamma ray having an energy level of 140.5 kilo electron Volts(keV) is detected. An energy level of 140.5 keV is typical of a gammaray photon generated by Technetium-99. However, because of a number offactors, including manufacturing tolerances and variations in electroniccomponent characteristics, an identically manufactured probe maygenerate an electronic pulse of 4.9 volts when detecting a gamma rayphoton having an energy level of 140.5 keV. Further, even if the outputof a particular probe is within acceptable tolerances at the factory,the output signal level may shift over time. When using probes whichvary over time or from probe to probe, the control units must,therefore, be calibrated using a known radioactive source so that theelectronic output signals from the probe are correctly interpreted bythe radiation detection system.

[0010] Radiation detection systems are typically calibrated against aradioisotope which has a known peak energy level. This may beaccomplished by, for example, calibrating each radiation detectionsystem periodically in a biomedical lab. The probe is held near aradioisotope having a known, characteristic, gamma radiation energylevel. Each gamma ray photon emitted by the radioisotope represents asingular radioactive event and each gamma ray photon has an energy levelmeasurable in kilo electron volts (keV). Each such gamma ray photon orradioactive event which is detected by a probe may be referred to as acount. Upon detecting gamma ray photons, the probe generates a series ofelectric pulses, each pulse having a voltage which is proportional tothe energy level of a gamma ray photon. Since the probe is positioneddirectly adjacent a radioactive source emitting gamma ray photons of aknown energy level, the number of counts associated with that energylevel would be far higher than the number of counts from other sourcessuch as background radiation or Compton scatter. Thus, with the probepositioned near a known source, the control unit may be adjusted tocalibrate the system by identifying the probe output signal (e.g.voltage) having the highest number of occurrences within a predeterminedtime period or by accepting a predetermined number of counts andidentifying the output signal (e.g. voltage) associated with the largestnumber of counts. The output signal associated with the largest numberof counts may then be interpreted to represent the energy level of thecalibration radiation source. In order to calibrate a probe properly astatistically significant number of counts must be used. The predominantpulse height, also called the peak pulse height, can be derived from therecorded spectrum of pulses. The peak pulse height is interpreted by theradiation detection system to correspond to the known, characteristicenergy level of the radioisotope used for calibration. Once the peakpulse height has been identified for a particular probe, the controlunit input window may be set to allow that signal to pass whilefiltering out other signals such as noise or Compton scatter.

[0011] Normally, probes are designed and manufactured to have apredetermined output signal level for a count of a predetermined energylevel. Unfortunately, a probe can lose calibration between the time itis calibrated in a lab and the time it is used on a patient. Calibrationloss (drift) can also occur due to mishandling of delicate probes orduring prolonged storage periods. In addition, the radioisotopestypically used in the calibration lab are not always the same as thoseused in the surgical patient (it is desired to inject a radioisotopewith a short half-life into the patient, whereas the half-life of theradioisotope used in the calibration lab is preferably long so that itcan be used over an extended period of time). Thus, the energy level ofthe radioisotope used to calibrate the radiation detection system may bedifferent then the peak energy level of the radionucleide injected intothe patient. Therefore, in current systems it may be necessary toprovide some means for extrapolating the results of the laboratorycalibration to the actual surgical situation. Although the individualcontributions probe drift, control unit drift, probe damage, and usingcalibration radioisotopes are typically small, it is desirable to reduceor eliminate them altogether. It would, therefore, be desirable tocalibrate the control unit to the output of a particular probeimmediately before its use, and preferably with the same radionucleideused in the patient. The latter approach would be practical if thephysician operator could perform the calibration immediately prior tothe procedure. The physician operator, however, often does not have theexpertise of a nuclear imaging technician, nor is the physician operatorworking in the controlled conditions of a biomedical laboratory. What isneeded is calibration method which could be easily performed by thephysician operator in the operating room. The calibration method couldbe made suitable for use by physician operators by automating many ofthe steps and by providing appropriate feedback signals to the operatorin order to properly position the probe during the calibrationprocedure. It would, therefore, be advantageous if the radiationdetection system could be calibrated using the radionucleide injectionsite. In particular, the injection site in a sentinel node procedure maybe suitable for use as a calibration source because most of the injectedradionucleide remains at the injection site for many hours afterinjection; the lymphatic system drains a relatively small amount duringthat time. Alternatively, it would be advantageous to design acalibration method which used a separate radioactive source available tothe physician operator, such as the radionucleide in the administrationsyringe which is available to the physician operator immediately priorto the surgery.

[0012] In addition to the need for the radiation detection system to beproperly calibrated, a filter is still required to discern radioactiveemissions of the radionucleide in suspect tissue from backgroundradiation. This background radiation, results predominately from Comptonscattering. Compton scattering (or scatter) results from the interactionof gamma ray photons with electrons of body tissues. The scattered gammaray photons have energies ranging from slightly below the full energygamma ray photons down to and below typical x-ray energies (the “Comptoncontinuum”). The apparent points of origin of these Compton scatteredgamma ray photons have only a limited relationship to the site fromwhich the unscattered, full energy gamma ray photons originated, andtherefore have little relationship to the location of the tissue ofinterest. When using the method disclosed in U.S. Pat. No. 5,732,704,much of this Compton scatter comes from the radionucleide concentratedaround the injection site in the patient, and this radiation can obscurethe gamma radiation emitted by the radionucleide that has migrated tothe sentinel node. Discerning the gamma radiation emitted by theradionucleide in the sentinel node from all other radiation sourcesreliably and consistently for each surgical patient is a highly desiredobjective of the surgeon. Therefore it is desirable to be able todiscriminate between those gamma ray photons having energies close tothat of the full-energy gamma ray photon and background radiation.Therefore, it is desirable to utilize a filter or window within thecontrol unit which eliminates the probe output signals representative ofbackground radiation. Normally the filter output includes the probeoutput signal levels representative of a full energy gamma ray photonand excludes probe output signal levels which are representative ofbackground radiation, including undesirable radiation resulting fromCompton scattering, such filters are known in the art.

[0013] The prior art discloses radiation detection devices that removebackground radiation using “windowing techniques” in order to discernand process the gamma radiation emitted by the radionucleideconcentrated in suspect tissue. U.S. Pat. No. 5,694,933 issued to Maddenet al on Dec. 9, 1997 discloses an apparatus having a hand-held probe, asignal processor, and a multichannel control unit (MCA) to identify apeak energy level, to set manually a window of energy levels, and toperform a variety of other functions.

[0014] Another phenomenon associated with radiation detection systems iscommonly known in the art as “pileup”. Pileup occurs when the frequencyof counts impinging on the forward window of the probe is higher thanthe response rate of the radiation detection system, especially thecrystal and detector portion of the probe. Thus the system is unable todetect and process each count individually. As a result, a multiplicityof counts emitted by an especially “hot” radiation source may bedetected as a smaller number of counts having a higher energy level.Pileup phenomena are of two general types, which have somewhat differenteffects on pulse height measurements. The first type is known as tailpileup and involves the superposition of pulses on the long-durationtail from a preceding pulse. A second type of pileup is called peakpileup and occurs when two pulses are sufficiently close together sothat they are treated as a single pulse by the radiation detectionsystem. These types of pileup lead to distortions of the recorded pulseheight spectrum and can cause a misinterpretation of the emissions ofthe radionucleide during both the calibrating of the probe and thelocating of the sentinel node. A detailed description of pileup isprovided in Radiation Detection and Measurement, by Glenn F. Knoll,pages 610-612, publisher John Wiley and Sons, Inc, (hereinafter Knoll).Knoll further describes electronic and statistical means for “pileuprejection” (pages 612-620) in order to reduce but not totally eliminatethe problems associated with pileup.

[0015] One method for reducing the effects of pileup is to position theprobe farther away from the radioactive source. Radiation intensity isinversely proportional to the square of the distance from the radiationsource. Therefore, the quantity of gamma ray photons from a particularsource which impinge on the receiving window of the probe is reduced bymoving the probe away from the source. It is difficult to properlycalibrate the probe if it is not positioned correctly with respect tothe calibration source. It would, therefore, be advantageous duringcalibration if a high-count, feedback signal is provided to the operatorwhen the count frequency is high enough to result in a significantpileup distortion of the recorded spectrum. Then the operator mayquickly reposition the probe-receiving window farther away from theradioactive source.

[0016] Another situation that may occur during calibration of aradiation detection system is when the operator does not position thereceiving window of the probe close enough to the radioactive source. Inaccord with the inverse square law for radiation propagation, theresulting count frequency may be very low. If the count frequency isvery low, the time required to detect a statistically significant numberof counts may be high (several seconds). When count frequency is so lowthat it would take a significant length of time to collect the requirednumber of counts, it would be advantageous to provide a low-count,feedback signal to the operator. Then the probe could be repositionedcloser to the radioactive source. Furthermore, if the low-count feedbacksignal is generated when the count frequency is less than desired, and ahigh-count feedback signal is generated when count frequency is morethan desired, then the operator is aided in positioning the probereceiving window the correct distance from the radioactive source.Finally, if a third feedback signal is generated when a desired countfrequency is obtained (neither too high or too low), it would be eveneasier for the operator to correctly position the probe relative to theradiation source. Thus, it would be advantageous to design a radiationdetection system having a calibration mode wherein the physicianoperator is assisted in positioning the probe during calibration of theradiation detection system. In such a system, the physician operatorcould calibrate the system during or immediately prior to initiating asurgical procedure.

[0017] The calibration positioning method described herein may becombined with an automatic windowing method for determining an energyacceptance window in order to reduce the effects of backgroundradiation. An operator could use the combined methods to calibrate theradiation detection system using the injection site in the patient ofthe radionucleide as the radiation source. By using such a calibrationmethod on the injection site, it would be practical to use low costprobes that generate a wide range of pulse magnitudes for counts of agiven energy level. Also, the effects of calibration error due toelectronic drift within the radiation detection system, damage to theprobe during handling, and calibration on a different radioisotope maybe diminished or avoided.

SUMMARY OF THE INVENTION

[0018] The present invention is a method of calibrating a radiationdetection system by adjusting the parameters of a control unit for eachprobe attached to the control unit. The method described provides areliable and practical way of using low cost radiation detection probeshaving variations in probe gain where probe gain is the ratio of voltagegenerated at the probe output for a given energy input. That is, probegain is the factor which relates the energy deposited in a probe by agamma ray photon to the voltage generated at the output of the probewhere the voltage is generated as a result of the photon striking thedetector input.

[0019] A method according to the present invention uses feedback toensure that the user has an appropriate count rate to accuratelycalibrate the system. The method comprises the following steps: Anoperator positions a radiation detection probe near a calibrationradiation source, such as the surgical injection site of aradionucleide. Then the radiation detection probe generates a pluralityof electronic pulses, each of the electronic pulses having a pulsemagnitude proportional to the energy level of a count. Next theradiation detection system calculates a pulse frequency. The radiationdetection system generates a low-count feedback signal when the pulsefrequency is less than a predetermined low-count frequency. Theradiation detection system generates a high-count feedback signal whenthe pulse frequency is greater than a predetermined high-countfrequency. The radiation detection system generates an optimal-countfeedback signal when the pulse frequency is greater than or equal to thepredetermined low-count frequency and is less than or equal to thepredetermined high-count frequency. Each of the feedback signals may bean audible feedback signal, a visual feedback signal, or both. When theprobe is positioned such that the optimal feedback signal is generated,the control unit begins to collect and record outputs from the probe.The control unit continues to collect and record output signals from theprobe until a statistically significant number of output pulses havebeen recorded. The control unit then categorizes the plurality of pulsesinto a plurality of pulse magnitude ranges. Next the control unitdetermines the number of pulses categorized in each of the plurality ofpulse magnitude ranges. The control unit then identifies the pulsemagnitude range containing the most pulses and labels that range thepeak pulse magnitude range. Then the system assigns an energy level toeach of the plurality of pulse magnitude ranges. The energy levelassigned to the peak pulse magnitude may be referred to as thecharacteristic energy level and is generally the characteristic energylevel of gamma ray photons emitted by the calibration source. Oncecalibrated, the control unit may use the characteristic energy level fora plurality of purposes. For example, the control unit may determine thelower cut off limits of an energy acceptance filter which includes thecharacteristic energy level but excludes pulses representative of gammaray photons having energy levels below a predetermined threshold energylevel. The threshold energy level generally has a predeterminedrelationship to the characteristic energy level. The radiation detectionsystem thereafter processes only pulses corresponding to an energy levelpassed by the energy acceptance filter. In another embodiment of thepresent invention, the energy acceptance filter also passes pulserepresentative of gamma ray photons having energy levels up to a highestenergy level having a second predetermined relationship to thecharacteristic energy level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The novel features of the invention are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and methods of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description, taken in conjunction with the accompanyingdrawings in which:

[0021]FIG. 1 shows a radiation detection system comprising a scanningprobe, a targeting probe, and a control unit;

[0022]FIG. 2 is a block diagram of a portion of the radiation detectionsystem shown in FIG. 1;

[0023]FIG. 3 is a sagittal section view of the breast of a surgicalpatient;

[0024]FIG. 4A is a first portion and

[0025]FIG. 4B is a second portion of a flowchart that describes a methodof calibrating a probe of a radiation detection system such as shown inFIG. 1;

[0026]FIG. 5 is an example of a pulse spectrum as it may be recorded bythe radiation detection system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0027]FIG. 1 is a perspective view of a radiation detection system 10 inaccordance with one embodiment of the present invention. Radiationdetection system 10 comprises a control unit 15, a scanning probe 36,and a targeting probe 37. Scanning probe 36 includes a scanning detectorassembly 28. Targeting probe 37 includes a targeting detector assembly29. Scanning probe 36 and targeting probe 37 may also be generallyreferred to as a probe 20. Probe 20 further comprises a probe housing22, a cable 24, and a connector 26. Probe housing 22 includes a modebutton 21 and an audio range switch 23. Audio range switch 23 includes arange up button 25 and a range down button 27. In FIG. 1, control unit15 further includes a volume set button 12, a volume knob 13, a cableinput 11, and a display 17. Display 17 includes a visual count rateindicator 18, a wait indicator 19, a range indicator 9, probe indicator83, left arrow 81, right arrow 82 and a battery indicator 14. A moredetailed description of a radiation detection system is provided in U.S.patent application Ser. No. 09/266,961 which was previously incorporatedherein by reference.

[0028]FIG. 2 is a block diagram of probe 20 and a block diagram of oneembodiment of probe 20 and control unit 15. In the block diagram of FIG.2, control unit 15 is in its calibration mode. Probe 20 compriseshousing 22 sealably containing a columnator 39, a detector 33 and apreamplifier/switch assembly 34. Probe 20 may be multi-patient usereusable and resterilizable instrument, or, alternately, be a singlepatient use disposable instrument. A suitable probe 20 may be purchasedfrom Neoprobe, Inc. as a Model 1017 14 mm reusable probe. Probe 20 mayalso be partially disposable such as, for example, when detector 33 isdetachable from probe 20 and is reusable/resterilizable and theremainder of probe 20 is disposable. Probe 20 is configured to bepositioned by a physician operator closely adjacent the site of aradiation source 30 which may be, for example, a radionelectrodeinjection site. Probe 20 detects gamma ray photons emitted by radiationsource 30 and other background radiation sources lying within afield-of-view 31 approximately centered on a longitudinal axis 32 ofprobe 20. Field-of-view 31 is also referred to as a “solid angle ofacceptance” and is established by the size, depth, and shape ofcolumnator 39.

[0029] One embodiment of preamplifier/switch assembly 34 of probe 20includes a printed circuit board (not shown), mode button 21, range downbutton 27 and range up button 25 are shown in FIG. 1.Preamplifier/switch assembly 34 amplifies each electronic signalreceived from detector 33 to a voltage magnitude that can be transmittedto control unit 15 via cable 24. All signals (including noise) fromprobe 20 are proportionally amplified and shaped into pulses in a linearamplifier 42 of control unit 15. In this embodiment, the resulting pulsemagnitudes are in the range of, for example, approximately 0-5 volts. Acount rate measurement circuit 43 counts the pulses generated by linearamplifier 42 over a time interval of, for example, 64 milliseconds. Asone example, count rate measurement circuit 43 may calculate a runningaverage of the pulses generated by linear amplifier 42 to generate aprevailing count rate. The prevailing count rate determined by countrate measurement circuit 43 increases dramatically when probe 20 ispositioned very close to radiation source 30, and conversely, decreasesto a very small rate when probe 20 is very far from radiation source 30.The distance between probe 20 and radiation source 30 should beoptimized during calibration to minimize effects detrimental to thecalibration process. As already described, detrimental effects includethe long time to obtain a statistically significant number of pulses forpeak identification where probe 20 is too far from radiation source 30and the phenomena associated with pileup where probe 20 is too close toradiation source 30. Therefore, control unit 15 is equipped with a meansto cue the operator as to positioning probe 20 in the optimal distancerange. A display driver 47 drives display 17 to provide the physicianoperator with visual cues. An annunciator driver 48 drives anannunciator 58 to provide the physician operator with audible cues.Either one or both of the visual and audible cues may be provided forusing the calibration method of the present invention. Count ratemeasurement circuit 43 is programmed to provide a first feedback signalto peak identification circuit 45, display driver 47 and annunciatordriver 48 when the measured count rate is below a predeterminedlow-count frequency. Measurement circuit 43 is programmed to provide asecond feedback signal to peak identification circuit 45, display driver47 and annunciator driver 48 when the measured count rate is above apredetermined high-count frequency. Control measurement circuit 43 isprogrammed to provide a third feedback signal to display driver 47 andannunciator driver 48 when the measured count rate is greater than orequal to the predetermined low-count frequency and is less than or equalto the predetermined high-count frequency.

[0030] The first, second, and third feedback signals may include visualcues. For example as illustrated in FIG. 1, during calibration, display17 may show a representation 83 of probe 20 positioned horizontally withreceiving end 38 pointed at a circle 84 representing a point radiationsource. The first feedback signal may include a left arrow 81 on display17 pointing towards the circle 84, thus cueing the operator to moveprobe 20 closer to the radiation source. The second feedback signal mayinclude a right arrow 82 on display 17 pointing away from circle 84,thus cueing the operator to move probe 20 away from radiation source 30.For the third feedback signal, the arrow on display 17 may disappear andwait indicator 19 may then appear, indicating the relative timeremaining to complete calibration while probe 20 is held an optimaldistance from radiation source 30. Similarly, audibly distinct tones maybe provided for each of the first, second, and third feedback signals.In one embodiment, a low-pitch intermittent tone is provided for thefirst feedback signal, a high-pitch intermittent tone is provided forthe second feedback signal, and a middle-pitch steady tone is providedfor the third feedback signal.

[0031] In FIG. 2, control unit 15 further includes a multi-channelanalyzer 44, also referred to as an MCA 44. A suitable MCA 44 may be aMicroace MCA which may be purchased from a EG&G Ortec, Inc. MCA 44stores in memory each pulse it receives from linear amplifier 42 duringthe sampling period and assigns each of those individual pulses to aparticular channel within MCA 44 based on the pulse magnitude (e.g.voltage). Each such channel is representative of a particular pulserange which, in turn is representative of a particularly energy range.As additional counts are detected by probe 20 they are distributed intothe various channels in MCA 44. This distribution of counts into variouschannels constitutes a spectrum or histogram of received pulsemagnitudes which are, in turn, representative of the energy content ofthe detected photons. The software in MCA 44 may be written specificallyto perform various functions including rejection of low level pulses,digitizing received pulses, and creation of a histogram of the receivedpulses. Suitable software is available from EG&G Ortec, Inc. as itsMARSTRO-32 MCA Emulator software. MCA 44 may include a microcontroller,and an analog-to-digital converter (ADC) for converting pulse voltages(for example, 0-5 volts) into digital numbers (for example, 0-4096 bits)that can be read by the microcontroller. MCA 44 may further include adiscriminator and reset delay device that is set by the microcontrollerto reject low level pulse magnitudes which result from electronic noiseor very low level radiation and to prevent pulses from reaching the ADCwhile it is converting an earlier pulse. With the discriminator andreset delay of MCA 44, the ADC processes pulses having magnitudes in therange of interest. The MCA 44 may also include a sample and hold deviceto hold a pulse during the time it takes to access the ADC.

[0032]FIG. 5 shows a typical spectrum plot 79 of energy levels of gammaray protons received by probe 20 and corresponding probe output voltagesfor a radioisotope such as Technetium-99. Spectrum plot 79 representsgraphically the accumulation of counts detected from the radioisotope byprobe 20 over a fixed time period, and includes a full-energy gamma raypeak 77 and a low level energy peak 73. Low level energy peak energypeak 73 may be caused by electronic noise and low level backgroundradiation. The nonzero spectrum between energy peaks is a result of anumber of factors, including Compton scattering. In FIG. 5, verticalaxis 70 represents the number of counts accumulated within a given timeperiod at a given energy. In FIG. 5, first horizontal axis 71 representsvoltage accumulated Second horizontal axis 72 represents the energylevel of accumulated counts in units of kilo electron volts (KeV).Full-energy gamma ray peak 77 has a maximum number of accumulated countsat pulse magnitude 76. The voltage (V_(p)) may, therefore, be associatedwith the known energy level (E_(p)). For example, the energy level ofgamma ray photons emitted by Technetium-99 is 140.5 keV. Therefore, ifTechnetium-99 is used as the calibration source voltage VP would beassigned to an energy level of 140.5 KeV. Once that assignment was made,output pulses from probe 20 having a voltage of VP would be interpretedby control unit 15 as having been generated by a gamma ray photon havingan energy level of 140.5 KeV.

[0033] The output of MCA 44 is provided to a peak identification circuit45. Peak identification circuit 45, which may be implemented in softwarein MCA 44, is designed to identify the characteristic full-energy gammaray peak. As an example, the software resident in the microcontroller ofMCA 44 which may be, for example, the MAESTRO-32 software mentionedpreviously, may be used to identify the full energy gamma ray peak Thepeak is identified by comparing the number of counts recorded in eachpulse magnitude range to the number of counts in the other pulsemagnitude ranges and selecting the pulse magnitude range or ranges withthe highest number of counts. Then the center point of the range orranges is determined and used as VP. Once a pulse magnitude is assignedto the energy of the gamma ray photons emitted by radiation source 30,the calibration is complete.

[0034] The output of the peak identification circuit 45 may be providedto a window setting circuit 46, which may also be implemented bysuitable software resident in control unit 15. Window setting current 46establishes the lowest and highest limits energy limits so as toestablish the width of the energy acceptance window encompassing thecharacteristic full-energy gamma ray peak. When the energy acceptancewindow has been set, a fourth feedback signal is provided to alert theoperator that the calibration of probe 20 is complete. The fourthfeedback signal may comprise, for example, a short audible beep. Fourthfeedback signal may also comprise a visual cue. For radiation detectionsystem 10 in FIG. 1, wait indicator 19 on display 17 indicatescompletion of the calibration method by changing to an “emptied”hourglass.

[0035] A physician operator may, therefore, quickly calibrate theradiation of electron system for a particular probe. To calibrate theradiation detection system the physician operator places the controlunit in calibration mode by, for example, processing one or more of thebuttons on probe 20. By using the feedback signals described previously,the operator may quickly and accurately position probe 20 an optimaldistance from radiation source 30 which is a known radioisotope. Sincethe positioning of probe 20 is quick and simple, it is possible for thephysician operator to calibrate the system immediately prior to asurgical procedure. Furthermore, the calibration may be performed usingthe radioisotope used in the surgical procedure. FIG. 3 shows a breast35 of a surgical patient. A probe 20 is positioned near an injectionsite 40 of a known radionucleide. The radionucleide containing, forexample, Technetium-99, has been injected near a tumor previouslylocated during a biopsy procedure. The radionucleide enters thelymphatic system surrounding the tumor and a portion of it migrates tothe sentinel node. A high concentration of the radionucleide remainsconcentrated around the injection site for at least several hours as theinjected radionucleide decays and is distributed throughout thepatient's body. During this time it is possible to obtain an accuratecalibration of probe 20 at the injection site using the calibrationmethod of the present invention.

[0036] The calibration method of the present invention is shown in aflowchart in FIGS. 4A and 4B. The first portion of the flowchart (FIG.4A) and the second portion of the flowchart (FIG. 4B) are divided at acircled letter “A”. The calibration method begins at Step 50 once theoperator has powered-up radiation detection system 10 and establishedthe radiation source (for example, the injection site of radionucleide)to perform the calibration. Next, in Step 51, the operator positionsprobe 20 near the radiation source such as shown in FIG. 3 and initiatesthe calibration mode by, for example, pressing one or more of thebuttons on probe 20 in a predetermined sequence. Probe 20 may be held bythe operator's hand, or may be fixed in a holding fixture havingadjustment features necessary to reposition probe 20 as required. InStep 52, probe 20 generates output pulses that are proportional to theenergy levels of the counts. In Step 53, count rate (also referred to aspulse frequency, PF) is calculated. The calibration method then proceedsto Step 54 to test whether the measured pulse frequency PF is less thana predetermined, minimal pulse frequency PF(MIN). A suitable value forPF(NIN) is approximately 100 counts per second, although PF(MIN) mayhave different values. If pulse frequency measured is less thanpredetermined minimal pulse frequency, then control unit 15 generates alow-count frequency feedback signal in Step 55. This alerts the operatorto position probe 20 closer to radiation source 30 or to redirectreceiving end 38 (see FIG. 2) so that the radiation source is within thefield-of-view 31 of probe 20. If pulse frequency PF is greater than apredetermined maximal pulse frequency PF(MAX) in Step 56, then controlunit 15 generates a high-count frequency feedback signal in Step 57. Theoperator may then reposition probe 20 farther from the radiation source.A suitable value for PF(MAX) is approximately 4000 counts per second,although PF(MAX) may have different values. If the measured pulsefrequency is in an optimal range for calibration as calculated in Step58, control unit 58 generates an optimal count range feedback signal.The operator then simply maintains the position of probe 20 untilcalibration is complete at Step 66. Once calibration is complete, andwhile probe 20 is positioned at an optimal distance from the radiationsource, control unit 15 automatically, within about 2 to 11 seconds forexample, performs steps 59-65 to determine and set an energy acceptancewindow.

[0037] In step 59 count rate measurement circuit 43 counts pulses fromlinear amplifier 42 and MCA 44 measures the pulse magnitude of eachpulse. The number of pulses counted is PM. Then in step 60,pre-programmed MCA 44 insures that a statistically significant number ofpulse magnitudes is recorded, or PM is greater than PM(MIN), wherePM(MIN) is a predetermined minimal number of pulse magnitudes. Step 60insures that enough pulses have been recorded such that when they areassigned to pulse magnitude ranges, enough data exists within each rangeto compare the number of pulse magnitudes in each range. As more pulsesare sampled, the error associated with the random distribution isreduced. A suitable value for PM(MIN) is approximately 1000 counts,although other values for PM(MIN) may be used with the presentinvention. In Step 61, the recorded pulses are grouped into channels orpulse magnitude ranges, thus forming a histogram as describedpreviously. Analytical methods for forming a histogram comprising aplurality of pulse magnitude ranges (sometimes called channels)representing energy levels are known in the art (see U.S. Pat. No.5,694,933). In the present invention, each pulse magnitude range has acommon, predetermined width depending on the full-energy gamma ray peakmagnitude of the radioisotope used for the calibration procedure. If theradioisotope used in the calibration is Technetium-99 having afull-energy gamma ray peak magnitude of 140.5 kiloelectron volts (keV),then a useful width of each pulse magnitude range is 2 keV, providingabout seventy pulse magnitude ranges. In Step 62, the number of pulsesin each pulse magnitude range is determined and in Step 63, a peak pulsemagnitude range (VP) is determined as described previously Peak pulsemagnitude range (VP) is the pulse magnitude range having the mostrecorded pulses.

[0038] Enhancements to the present invention will occur to those skilledin the art. For example, a calibration method according to the presentinvention may use a number of qualifiers to ensure that the selectedpulse magnitude range is representative of a true full-energy gamma peak(having a magnitude easily discernible from the rest of the spectrumplot). In general, a qualifier is a set of conditions programmed intoMCA 44, wherein each pulse must meet the set of conditions to be furtherprocessed. Qualifiers would be advantageous during the calibrationmethod if, for example, probe 20 was not pointed directly at theradiation source, but pulse frequency was high enough to proceed withthe method. If probe 20 was not pointed at the radiation source, none ofthe full-energy gamma ray photons would impinge directly on detector 33,and therefore control unit 15 would not accurately record a full-energypeak but might record a high number of counts in a particular range,giving a false peak. Rather, a relatively flat spectrum plot would beobtained, except for the peaks associated with noise and backgroundradiation. In such a situation, if the histogram did not fit apredetermined characteristic curve the operator could be alerted (byaudible or visual feedback signals) to redirect probe 20 so thatradiation source 30 is within field-of-view 31 of probe 20.

[0039] In step 64 of FIG. 4B, the known, characteristic, full-energylevel for the radioisotope used in the calibration is assigned tocorrespond with the center voltage in the peak magnitude range. Theremaining pulse magnitude ranges are then assigned lower energy levels,although counts falling within these lower energy pulse magnitude rangesare not necessarily weighted as equal to the full energy level pulsemagnitude ranges. A non-proportional assignment of weighting countswithin these pulse magnitude ranges may be used, for example, toincrease the sensitivity to higher energy level pulse magnitudes, thusimproving the ability of radiation detection system 10 to discern fromwhich direction the detected radiation of interest is coming. Once Step65 is complete, the system calibration is complete.

[0040] Once the system calibration is complete, other system parametersmay be set. In step 65 for one embodiment (A) of the present invention,control unit 15 automatically sets an energy acceptance window havingonly a lowest energy level L (see FIG. 5) having a predeterminedmathematical relationship to the peak energy level P. For example, thelowest energy level L may be set to be a predetermined percentage of thepeak energy level P. For the radiation detection system of FIG. 1, alowest energy level L that is, for example, about 78.2% of the peakenergy level P approximately represents detection of only those gammaray photons impinging at less than a 90 degree angle to field-of-view31.

[0041] In step 65 for another embodiment (B) of the present invention,control unit 15 automatically sets an energy acceptance window havingboth a lowest energy level L and a highest energy level H (see FIG. 5),each having a predetermined mathematical relationship to the peak energylevel P. This embodiment may be desirable, for example, if aradioisotope with multiple energy peaks is being surveyed.

[0042] While preferred embodiments of the present invention have beenshown and described herein, it will be obvious to those skilled in theart that such embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. Accordingly, it isintended that only the spirit and scope of the appended claims limit theinvention.

What is claimed is:
 1. A method for calibrating a radiation detectionsystem for processing and analyzing counts from a radiation source,whereby a count is a single radioactive event having an energy level,said method comprising the steps of: a) positioning a probe of saidradiation detection system near said radiation source; b) generating aplurality of electronic pulses, each of said electronic pulses having apulse magnitude proportional to the energy level of a count detected bysaid probe; c) counting said plurality of electronic pulses andcalculating a pulse frequency; d) generating a low-count feedback signalwhen said pulse frequency is less than a predetermined low-countfrequency; e) generating a high-count feedback signal when said pulsefrequency is greater than a predetermined high-count frequency; f)generating a optimal-count range feedback signal when said pulsefrequency is greater than or equal to said predetermined low-countfrequency and is less than or equal to said predetermined high-countfrequency; g) measuring a plurality of pulse magnitudes when saidoptimal-count range feedback signal is provided; h) grouping saidplurality of pulse magnitudes into a plurality of pulse magnituderanges, and each pulse magnitude range has a width; i) determining thenumber of pulse magnitudes corresponding to each of said plurality ofpulse magnitude ranges; j) determining a peak pulse magnitude range fromsaid plurality of pulse magnitude ranges, whereby said peak pulsemagnitude range has the maximum number of corresponding pulsemagnitudes; k) assigning an energy level to each of said plurality ofpulse magnitude ranges, whereby a peak energy level having a known,characteristic value according to said radiation source is assigned tosaid peak pulse magnitude range, and remaining pulse magnitude rangesare assigned energy levels that are lower than said peak energy level;and l) setting an energy acceptance window comprising a continuousportion of said energy levels containing said peak energy level, saidenergy acceptance window containing a lowest energy level having a firstpredetermined relationship to said peak energy level, whereby saidradiation detection system thereinafter processes only pulsescorresponding to an energy level falling within said energy acceptancewindow.
 2. The method of claim 1 wherein said energy acceptance windowcontains a highest energy level having a second predeterminedrelationship to said peak energy level.
 3. The method of claim 1 whereinsaid low-count feedback signal comprises an audible feedback.
 4. Themethod of claim 1 wherein said low-count feedback signal comprises avisual feedback.
 5. The method of claim 1 wherein said high-countfeedback signal comprises an audible feedback.
 6. The method of claim 1wherein said high-count feedback signal comprises a visual feedback. 7.The method of claim 1 wherein said optimal-count range feedback signalcomprises an audible feedback.
 8. The method of claim 1 wherein saidoptimal-count range feedback signal comprises a visual feedback.
 9. Themethod of claim 1 wherein said predetermined low-count frequency isapproximately 100 counts per second.
 10. The method of claim 1 whereinsaid predetermined high-count frequency is approximately 4000 counts persecond.
 11. The method of claim 1 wherein said plurality of counts is apredetermined plurality of counts.
 12. The method of claim 11 whereinsaid predetermined plurality of counts is approximately 1000 counts. 13.The method of claim 1 wherein said width of each of said pulse magnituderanges is a predetermined width.
 14. The method of claim 13 wherein saidpredetermined width of each of said pulse magnitude ranges isapproximately in the range of 1-5 Kilo-electron Volts.
 15. The method ofclaim 1 wherein said first predetermined relationship of said lowestlimit energy level to said peak energy level is a predeterminedpercentage of said peak energy level.
 16. The method of claim 15 whereinsaid predetermined percentage of said peak energy level is approximatelyin the range of 70-90 percent.